I have the same problem!
I have 2 RAID (1-ssd, 2-hdd).
mobo --- ASUS Gryphon Z87.(raid rom 188.8.131.52.2164)
hdd --- 2x SanDisk Ultra Plus 128GB [Raid1] + 2x WD4000FYYZ [Raid1].
At the same time, on the ssd array I got acceleration reading, and on the HDD array - no!
HDD RAID 1
SSD RAID 1
As can be seen, raid 1 on ssd works as it should and gives almost a x2 read speed compared to 1 ssd. But with hdd - No!
Tested on multiple platforms (z77, z87, z97) - everywhere raid 1 does not work correctly with hdd, but ssd - works fine (that is, the array gain in reading speed !!)
The same situation with an array in HP proliant mickoserver gen 8 server (has renamed intel controller).
Here is a test of the old disks (80GB) in raid 1 and raid 0 + ASUS Gryphon Z87.(raid rom 184.108.40.206.2164):
As we see the increase in reading speed of RAID 0 was almost 2x faster, while RAID 1 - reading speed at 1 disc.
From the tests carried out by me, we can conclude that the controllers in the intel chipsets with support raid 1 do not work properly with HDD drives!
Intell Company specifically trims speed raid 1 (hdd) in a custom version iRST (problem in raid driver !!! raid rom iRSTe no need for iRSTe driver installation), then, in the version iRSTe (enterprise) reading speed increase in raid1 !!!
Forcing Intel RSTe RAID Drivers 220.127.116.112 driver (via Device Manager and bypassing the compatibility test) help increase your reading speed! Here's a screenshot: